THE JOURNAL OF CHEMICAL PHYSICS 139, 174703 (2013)
A first principle study for the adsorption and absorption of carbon atom and the CO dissociation on Ir(100) surface I. A. Erikat1,a) and B. A. Hamad2 1 2
Department of Physics, Jerash University, Jerash-26150, Jordan Department of Physics, The University of Jordan, Amman-11942, Jordan
(Received 9 July 2013; accepted 16 October 2013; published online 5 November 2013) We employ density functional theory to examine the adsorption and absorption of carbon atom as well as the dissociation of carbon monoxide on Ir(100) surface. We find that carbon atoms bind strongly with Ir(100) surface and prefer the high coordination hollow site for all coverages. In the case of 0.75 ML coverage of carbon, we obtain a bridging metal structure due to the balance between Ir–C and Ir–Ir interactions. In the subsurface region, the carbon atom prefers the octahedral site of Ir(100) surface. We find large diffusion barrier for carbon atom into Ir(100) surface (2.70 eV) due to the strong bonding between carbon atom and Ir(100) surface, whereas we find a very small segregation barrier (0.22 eV) from subsurface to the surface. The minimum energy path and energy barrier for the dissociation of CO on Ir(100) surface are obtained by using climbing image nudge elastic band. The energy barrier of CO dissociation on Ir(100) surface is found to be 3.01 eV, which is appreciably larger than the association energy (1.61 eV) of this molecule. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4827516] I. INTRODUCTION
The formation of graphene on transition metal surfaces is interesting to gain more information about the catalytic growth of single-wall carbon nanotubes (SW-CNT’s). Thus, the adsorption, diffusion, migration, and bond formation of carbon atoms on or in metallic catalyst should provide useful information to the manipulation and the design of carbonbased nanomaterials.1–3 It is well known in the literature that Ir(100) surface exhibits a quasi hexagonal reconstruction with (1 × 5) periodicity,4, 5 where the (1 × 1) unreconstructed overlayer is stable up to temperature >800 K.5 The adsorption of some small molecules, such as CO, NO, O2 , and H2 , can lift the reconstruction.6, 7 On the other hand, the adsorption of small adsorbates such as O, C, and N causes reconstruction.6, 8 Johnson et al.9 studied the structure of carbon adsorbed on Ir(100) surface using low energy electron diffraction and density functional theory. They found that carbon forms a c(2 × 2) overlayer and occupies the fourfold hollow site but does not cause the surface to clock reconstruct as found for Ni(100) by Alfe et al.,8 due to the iridium high cohesive energy. The dissociation of CO is very important in the production of hydrocarbons from carbon monoxide and hydrogen by Fischer-Tropsch (FT) synthesis and in the formation of higher oxygenates.10–12 Moreover, CO dissociation is the rate determining and the first step in many catalytic processes and is an essential step for getting rid of toxic substances that are produced during the burning of the fuel.13 It is wellknown that nickel, cobalt, iron, and ruthenium are very active as FT catalysts, while platinum, palladium, and iridium exhibit low activity.14 There are several studies for the case of a) Author to whom correspondence should be addressed. Electronic mail:
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the active elements for CO dissociation, whereas, the studies for the less active catalysts are scarce. Liu and Hu15–17 performed DFT studies to calculate the dissociation of CO on series of flat, stepped, and kinked 4d as well as 5d closepacked metal surfaces, namely Ru(0001), Rh(111), Pd(111), Os(0001), Ir(111), and Pt(111) at 0.25 ML coverage. They found similar late transition states for all these surfaces. In addition, they found that the effect of decreasing the coverage to 1/9 ML is minor on the calculated energy barrier. For example, the energy barrier of CO dissociation on Rh(111) changed from 1.25 to 1.17 eV by decreasing the coverage from 1/4 ML to 1/9 ML.17 There are many experimental and theoretical studies for the adsorption of CO on Ir(100) surface, where CO is found to adsorb linearly (vertically) on Ir(100) surface with C atom pointing towards Ir surface. The top site is found to be the most stable site for coverages 0.25 ML and 0.50 ML.7, 18, 19 On the other hand, there are few studies in the literature about the dissociation of CO molecule on Ir surfaces.13, 15 It was found that the dissociation reactions are affected by two factors; the first one is the electronic factor, where the reactivity of transition metal dissociation reactions decreases from left to right in the Periodic Table.20, 21 The second factor is the geometrical one due to the relative change in the molecular-substrate interaction strength on low coordinated atoms.21–23 However, there are many details that remain unclear even after significant studies regarding the structure-sensitivity, the details of the reaction path, and the bonding coordination of reaction intermediates to the surface. The aim of the present study is to analyze the electronic factors that determine CO dissociation on Ir(100) surface. The motivation is twofold, one is environmental to reduce CO emission and the other is industrial to understand the effect of carburizing the Ir surface on its catalytic reactivity.
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II. CALCULATION METHOD
III. RESULTS AND DISCUSSION
We performed first-principles calculations based on DFT as implemented in Quantum espresso Package24 that solve Kohn-Sham equations using a plane-wave basis set. For the treatment of electron exchange and correlation, we use the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE).25 The electron-ion interactions are described with ultrasoft pseudopotentials.26 We use our previous calculated lattice constant, 3.899 Å,18 which is ∼1.7% higher than the experimental value (3.84 Å)27 to build our supercells. The surface is modeled using a 2 × 2 unit cell with a five-layer symmetric slab separated in the z-direction by a vacuum thickness of 15 Å. The adsorbates are placed on one side of the slab. The center of the slab consists of a mirror plane where the atoms are fixed at their bulk positions, whereas the layers above and below as well as the adsorbates are freely allowed to relax in all directions. The advantage of using this symmetric model is due to the negligible dipole correction28, 29 as we found a minor change of about 0.01 eV/C in the adsorption energy. A cut-off energy of 680.5 eV and Methfessel Paxton broadening with σ = 0.2 eV30 are used. The Brillouin zone is sampled with the Monkhorst-Pack k-mesh31 (6 × 6 × 1). The reference energy of the gas phase for C, O, at their triplet ground state, and CO are calculated by placing the atom or molecule in a cubic cell with a side length of 15 Å and carrying out a spin-polarized gamma-point calculations. The criteria for the converged calculations are 10−7 eV for the energy and less than 10−3 eV/Å for the maximum Hellmann-Feynman forces acting on each atom upon ionic relaxation. Carbon atoms are allowed to reach the surface at different surface sites (top, bridge, and hollow) and two different subsurface sites (tetrahedral and octahedral). The adsorption and absorption energies per carbon atom, Eads and Eabs , respectively, are both referenced to isolated carbon atoms in the gas phase and a pure iridium slab by the formula32
A. Carbon adsorption on Ir(100) surface
Eads/abs = −(Etot − EI r − mEC(g) )/m,
The adsorption of C atom on Ir(100) surface is investigated at the coverages 0.25 ML, 0.50 ML, 0.75 ML, and 1.00 ML. For all coverages the energetically most favorable adsorption site is the hollow site, which is consistent with previous studies of the adsorption of carbon atoms on (100) faces of other fcc transition metals such as Ni(100) and Rh(100).8, 35, 36 The adsorption energy of C on Ir(100) surface as a function of coverage is plotted in Fig. 1. The adsorption data indicates that C atoms bind strongly with Ir(100) surface preferring the high coordination hollow site. The adsorption energy decreases significantly for coverages greater than 0.25 ML, which indicates that a strong repulsive interaction builds up between adsorbates. The adsorption energy at 0.25 ML is 7.87 eV, which is more energetically stable than that for bridge and top sites by 1.14 eV and 2.31 eV, respectively. For this coverage at the hollow site, the Ir–C and Ir– Ir bond lengths are 2.06 Å and 2.78 Å, respectively, which means that the Ir–Ir distance beneath C atom increases by 0.02 Å from the original interatomic distance. In addition, Ir second layer atoms suffer from buckling of 0.06 Å. The outermost Ir atom at Ir(100)-C system with carbon atom at hollow site contracts by 0.59% relative to the unrelaxed clean surface interlayer spacing (d0 ). Moreover, the second layer contracts by 0.66%. However, by comparing this with the clean Ir(100)(2 × 2) surface,18 one can conclude that the adsorption of carbon atom on the hollow site at Ir(100)-C system decreases the contraction of the first layer by 5% and removes the expansion of the second layer. At 0.5 ML the hollow site is energetically the most stable site with adsorption energy of 7.73 eV, which is greater than those at the bridge and hollow sites by 1.11 and 2.16 eV, respectively. Our calculated adsorption energy at the hollow site is smaller by 0.46 than that obtained from LEED experiment (8.19 eV)9 and 0.33 eV from DFT calculations by Johnson et al. (8.04 eV)9 and Yamagishi et al. (8.06 eV).37 Table I represents a comparison between the energetic and
(2.1)
where Etot refers to the total electronic energy of the Ir(100) − mC system, EIr is the energy of clean Ir(100) surface, EC(g) refers to C electronic energy in the gaseous phase, and m is the number of C atoms on/in the surface. The generalized cohesive energy (Ecoh )33 is defined as Ecoh (AB) =
Etot (AB) − mA Eat (A) − mB Eat (B) , mA + mB
(2.2)
where Etot (AB) is the total energy of the solid and Eat (A, B) is the total energy of non-spin-polarized atoms. The effect of spin polarization is tested for all systems and found to be negligible. The climbing image-nudged elastic band (CI-NEB) method34 is used to locate the minimum energy paths (MEP) and the transition states.
FIG. 1. The adsorption/absorption energy per carbon atom as a function of C coverage (); top (t), bridge (b), hollow (h), and octahedral (oct).
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TABLE I. A comparison of the results in this work for the energetic and structural parameters of Ir(100)-(2 × 2)-2C system with LEED and previous DFT calculations.8, 39 Eads : the adsorption energy for carbon atom; Ir–C: iridium-carbon bond; dij : the relative interlayer distance between the ith and jth layers; z: second layer buckling; dIr–C : the vertical distance between the carbon atom and the first iridium layer.
This work DFTa LEEDa a
Eads (eV/C)
Ir–C (Å)
d12 (%) (%)
d23 (%)
z (Å)
dIr–C (Å)
7.73 8.04 8.19
2.06 2.04 2.05
3.53 4.40 2.70
−1.43 n/a −1.80
0.00 n/a 0.02
0.62 0.68 0.74
Reference 9.
structural properties in our work and previous experimental and DFT calculations of Ir(100)-(2 × 2)-2C system. The discrepancy between our calculations and the previous DFT calculations can be related to the difference between exchange correlation functionals used in both studies38 as well as the type of surfaces used. In our calculations we used the symmetric model for the surface, whereas those previous DFT calculations were performed using the asymmetric one, see Sec. II. Moreover, we did not include the reconstruction energy per (1 × 1) cell for the (1 × 5) →(1 × 1) phase transition of the clean surface as it is the case of Yamagishi et al.37 The Ir–C bond does not change by increasing the coverage and there is no buckling in the first and second layers. The outermost Ir layer expands by 3.5% relative to d0 , whereas the second layer contracts by 1.4% relative to d0 . At 0.75 ML coverage one of the three carbon atoms penetrates into the subsurface with two Ir–C bond lengths of 2.04 Å and 1.94 Å that creates bridge metal (BM) structure, see Fig. 2. This BM structure was also found to be spontaneously formed on Cu, Ag, and Ni surfaces.39 For this BM structure, we obtained an adsorption energy of 6.96 eV/C and a cohesive energy of 7.49 eV. Therefore, the relative strength (the ratio between the adsorption energy and the cohesive energy) is found to be 0.93, which is slightly smaller than that obtained by Wu et al. (1.00) for Ir(111) surface.39 However, we obtained a spontaneous BM structure at the Ir(100) surface unlike their findings for Ir(111). We can relate this to the fact that the Ir(100) surface is an open surface, whereas Ir(111) is the most compact surface for the fcc structure with smaller M-M bond
(a)
(b)
FIG. 2. (a) Top view and (b) side view of the BM structure at relaxed Ir(100)3C system.
length, see Fig. 2. The first layer is found to expand upwards by 7.3% relative to d0 with lateral motions of the atoms in the x and y directions away from each other by 0.04 Å. At the second layer, the iridium atoms below the carbon atoms move upward by 1.82% relative to d0 , whereas the fourth iridium atom is pulled upward by 9.6% relative to d0 . As the carbon coverage increases to 1.00 ML, the adsorption energy per carbon atom decreases to 5.99 eV and no carbon atoms penetrate into the surface unlike the case of the three monomer (0.75 ML). The existence of BM at 0.75 ML and not at 1.0 ML is related to the competition between Ir–C and Ir–Ir interactions. If the Ir–C interaction is stronger than that of Ir–Ir, the Ir atom below C will be up shifted, which leads to an increase of the coordination numbers of carbon atoms. This is not the case for the present system since Ir–Ir interaction is stronger than Ir–C. In order to confirm the validity of using the five-layer slab in the calculations, we used a larger slab of seven layers, where the three layers above and below the central layer are allowed to relax. We found that the adsorption energies for all coverages, with carbon atoms at the hollow site, have increased by 0.08 eV as compared to the values for the case of the five-layer slab. The percentage difference of the interlayer spacings with respect to the bulk values for the third layer is found to be negligible for all coverages. We also found that the effect of spin polarization is negligible for Ir(100)-C system at all coverages. Figure 3 represents the projected density of states (PDOS) for carbon atom adsorbed on the hollow site and the nearest neighbor Ir atom for the 0.25 ML, 0.50 ML, 0.75 ML, and 1.0 ML coverages. The adsorption of C on Ir(100) surface shifts the d-band center below the Fermi level relative to its location at clean surface by 0.40 eV, 0.85 eV, 1.24 eV, and 1.90 eV, for 0.25 ML, 0.50 ML, 0.75 ML, and 1.00 ML coverages, respectively. This means that the higher the d-band center relative to the Fermi level, the stronger the bonds between Ir and C atoms, which is consistent with the Hammer-Nørskov d-band model.40 The broadening of C 2p orbital increases with increasing the coverage due to the increase in the hybridization with Ir 5d orbital. There are overlaps between 2s + 2p for C atom and 5p + 5d for Ir atom from −14 to −11 eV. In the case of 0.25 ML coverage there is a strong hybridization between Ir 5d and C 2p orbitals near 2.20, 1.40, −7.60, −6.70, −5.30, −4.30, and −2.00 eV. As the coverage increases, the hybridization between d and p bands increases and the peaks in the antibonding region shift to the left. Moreover, for the 0.50 ML coverage, the peaks of Ir 5d orbital are well localized at −7.60, −7.00, −6.40, −5.30, −4.80, −4.3, −3.30, −2.67, 1.00, and 2.28 eV. This behavior indicates small repulsive interaction between the C atoms. In Fig. 3(d) (0.75 ML), one can notice the charge transfer from Ir 5d orbital to the 2p orbital of the carbon atom in BM structure, which decreases the density of state below the Fermi energy of the Ir 5d, while adding features to the energy of 2p level of the adsorbate (carbon atom). This leads to sharp peaks at −8.30, −7.63, −6.60, 1.47, 0.76, and 4.16 eV due to the strong hybridization between the 2p orbital of the carbon atom and Ir 5d. At the 1.00 ML coverage, the peaks become sharper at −12.10, −7.10, −6.60, −4.60, −3.8, −2.74, −1.70, 0.41, and 1.48 eV. From the above
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FIG. 3. (a) PDOS of the 5d orbital of Ir(100) clean surface. (b) PDOS of Ir (100)-C system with coverage 0.25 ML. (c) PDOS of Ir (100)-2C system with coverage 0.50 ML. (d) PDOS of Ir (100)-3C system with coverage 0.75 ML. (e) PDOS of Ir (100)-4C system with coverage 1.00 ML. Solid black line is Ir 5d, red dotted line is C 2p orbital for C atom at hollow site. The blue dashed line at (d) is C 2p for the BM carbon atom at Ir(100)-3C. The vertical dashed and solid lines denote the Fermi level and the center of the d band, respectively.
results we can argue that the adsorption of carbon atom on Ir(100) surface is coverage dependent.
B. Carbon absorption in Ir(100) surface
In this subsection we present the results of carbon absorption in Ir(100) surface. We find that carbon atom prefers the octahedral site, in the subsurface, see Fig. 4, with absorption energy of 5.39 eV, which is greater than that of the tetrahedral site by 0.53 eV. The absorption energy increases by 0.18, 0.42, and 0.49 eV when increasing the coverage to 0.50, 0.75, and 1.00 ML, respectively, see Fig. 1. From Fig. 1, one can see that at the 1.0 ML coverage, the difference between the adsorption energy at the hollow site is very close to the
FIG. 4. The most stable structure of C atom in Ir(100) subsurface. Ir atoms are in gray color and C atom is in yellow color.
absorption energy at the octahedral site. This means that the diffusion energy barrier at this coverage is very small. We also investigate the effect of increasing the carbon content of the (2 × 2) unit cell from one atom per unit cell up to four atoms and changing the relative amount of carbon on the surface and in the subsurface. In order to reduce the number of studied configurations, we consider only the configurations with highest adsorption/absorption energies (hollow site on the surface and octahedral in subsurface), see Fig. 5. This procedure can show the energetically preferred states and the energy barriers to move from one state to another. Figure 5 shows that the on surface position is favored for coverages up to 0.50 ML (2 carbon atoms per (2 × 2) cell). However, for 3 carbon atoms per (2 × 2) unit cell (0.75 ML coverage), the optimum structure is obtained when two of the three carbon atoms are located at the hollow sites and the third at the octahedral below the surface. The adsorption/absorption energy of this configuration is 7.08 eV/C, which is greater by 0.12 eV/C than the case with three carbon atoms adsorbed at the hollow sites. Moreover, in the case of four carbon atoms per (2 × 2) unit cell (1.00 ML coverage), the most stable structure is found when two carbon atoms are located at the hollow sites with an adsorption/absorption energy of 6.78 eV. The stability of this configuration is almost the same as the case when three carbon atoms are adsorbed at the hollow sites and the fourth in the octahedral site, with an energy difference less than 0.01 eV/C. The decrease of the stability at the higher coverages when all atoms are adsorbed on the surface can be explained by the increase of the repulsive interaction between carbon atoms on nearest neighbor sites.
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FIG. 5. The energy diagram of the adsorption/absorption energy per carbon atom on/in surface/subsurface of Ir(100)-(2 × 2). Cads represents adsorbed carbon atoms on the hollow site and Csub subsurface carbon on the octahedral site.
C. Carbon diffusion into Ir(100) surface
We used the CI-NEB method to obtain the minimum energy path of carbon diffusion into the Ir(100) surface. Figure 6 shows the minimum energy path of C diffusion into Ir(100) surface at 0.25 ML coverage. The energy change from the reactant to the product is 2.48 eV. The reaction is endothermic, which means that the carbon atom prefers to stay on the surface. We obtain a diffusion barrier of 2.70 eV going into the surface, while the reverse process has a barrier of 0.22 eV. Our finding for the diffusion barrier is much larger than the case of Ir(111) (0.75 eV).41 The reason of the high diffusion barrier obtained in our calculations as compared to the case of Ir(111) is referred to the higher difference between the adsorption and absorption energies of 2.48 eV/C versus
FIG. 6. The minimum energy path of C diffusion into Ir(100) and the structures for the initial, two intermediate, transition, and final states. Ir atoms are in gray and C in yellow.
1.78 eV/C for Ir(111).41 At the transition state, the carbon atom resides in an approximate tetrahedral site making two Ir–C bonds one with Ir surface atom and another with Ir in the second layer, see Fig. 6. Consequently, one Ir surface atom moves up towards the vacuum. According to the small energy difference of 0.53 eV/C between the subsurface at the tetrahedral site and the subsurface octahedral site, the diffusion of the carbon atom from the subsurface to the surface is rather easy.
D. The dissociation of CO molecule on Ir(100) surface
The adsorption of O and CO was studied extensively in our previous works.18, 42–44 At the 0.25 ML coverage, we found the bridge and top sites to be the most energetically stable sites with adsorption energies 5.20 eV and 2.37 eV, for O and CO, respectively. The dissociation energy for the free CO molecule is found to be 11.81 eV, which is in a fair agreement with the experimental value of 11.16 eV45 as well as previous GGA calculations of 11.31 eV.46 In this subsection, we investigate the dissociation of CO molecule on Ir(100) surface using the most strongly bound molecular state (top) as an initial state (IS) and the most stable final state (FS) for Ir(100)-(C + O) system, with C and O at the hollow and bridge sites, respectively. Seven intermediate images are suggested between the initial and final states to search for the reaction path using CI-NEB. Figure 7 shows the energy change along the minimum pathway and Fig. 8 shows snapshots of the initial, intermediate, and final states along the calculated minimum energy reaction path. The reaction is endothermic, with heat of reaction (H) of 1.40 eV between the IS and FS. The activation barrier (Ea ) of the dissociation reaction is 3.01 eV, which is large as compared to the barrier of the association reaction for C + O → CO (1.61 eV). The activation energies for CO dissociation depend strongly on the nature of the metal and
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FIG. 7. The minimum energy path for CO dissociation on Ir(100) surface as determined from CI-NEB calculations. Ea is the dissociation barrier and H is the heat of reaction.
is highly structure sensitive.13 Liu et al. found an activation energy of 1.47 eV for the CO reaction on Ir(111) surface15 by using constrained minimization method. In our previous study on CO oxidation on Ir(100) surface43 we found two different TSs and reaction paths by using constrained minimization and nudged elastic band methods. The energy barrier associated with TS, using the constrained minimization and NEB methods are 0.85 and 1.34 eV, respectively. The energy barrier obtained by constrained minimization method is smaller than that obtained by the NEB due to the different reaction paths of each method. van Santen et al. estimated the transition state energies for CO dissociation on several (111) metal surfaces
(a)
(f)
(b)
(j)
(c)
(h)
(d )
(e)
from the Brønsted–Evans–Polanyi formula and found an energy barrier of 3.48 eV (336 kJ/mol) at Ir(111) surface.13 The dissociation process starts when carbon monoxide at the top site inclines with a tilt angle of 2.34◦ . This inclination is expected because previous studies reported that perpendicularly adsorbed CO desorbs molecularly without dissociation at high temperatures,21 whereas the inclined CO is believed to dissociate due to strong interactions with the d-band of the substrate metals that causes the weakening of the C–O bond. At the second step carbon atom diffuses to the bridge site that has a smaller adsorption energy than the top site (2.20 eV) making two bonds with Ir atoms; 2.06 Å and 2.09 Å. Therefore, the C–O bond increases by 0.02 Å as compared to C–O bond at the IS (1.16 Å). At the next event, the carbon atom diffuses towards the hollow site and makes threefold bonding with the surface with Ir–C bond lengths about 2.17 Å. However, the fourth Ir atom has a larger Ir–C distance of 2.39 Å. At this metastable state, there is an increase in the C–O bond by 0.11 Å as compared to the same bond at IS, see Fig. 9(a). At this configuration, there is a buckling in the first layer as well as the second layer beneath CO molecule of
(i)
FIG. 8. Snapshots for the initial, seven intermediate, and final states along the dissociation energy path for CO on Ir(100) surface. (a) The initial state. (b), (c), (d), (e), (g), and (h) Intermediate images. (f) The transition state. (i) The final state. Dark gray balls represent Ir first layer atoms, light gray balls represent Ir second layer atoms, yellow balls represent C atoms, and red balls represent oxygen atoms.
(a)
(b)
FIG. 9. Side views CO dissociation reaction at (a) the intermediate state just before TS. (b) TS.
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FIG. 10. PDOS for three states at CO dissociation on Ir(100) surface: (a) IS, (b) TS, and (c) FS. The solid black line denotes Ir 5d band, the green dotted line denotes O 2p orbital, and red dashed line denotes C 2p orbital.
0.04 Å and 0.06 Å, respectively. At the transition state (TS), the carbon atom diffuses to the hollow site, which is the most stable site and oxygen atom moves to the nearest top site, see Fig. 9(b). At this state the three Ir–C bond lengths are 2.08 Å, 2.02 Å, and 2.02 Å, whereas, the Ir–O bond length is 1.88 Å. Finally, the carbon and oxygen atoms diffuse to the fourfold hollow and the bridge sites, respectively. The scenario of CO dissociation in our calculations has several features, first: CO diffuses from the most stable site (top) of adsorption energy of 2.37 eV to a less stable site (bridge) of adsorption energy of 2.20 eV and then to the most unstable site (hollow) of adsorption energy of 1.96 eV. As CO reaches the hollow site, the CO bond becomes weaker and the bond length increases by 0.11 Å. At TS, oxygen atom diffuses to the most unstable site, which is the top site. However, the carbon atom diffuses to the most stable site (hollow site). In addition, at TS, the distance between C and O is large, which is usually called ‘late TS.’’ This is consistent with the results found for other transition metal surfaces.15, 21, 22 The adsorption of CO molecule on Ir(100) surface lowers the d-band center from −2.7 eV to −3.52 eV. Figure 10 shows the distribution of the projected density of d-states of the Ir(100) surface as well as the p-states of C and O atoms. From Fig. 10(a), one can notice that there is a charge depletion from 5σ to Ir 5d band, also there is a back donation from Ir 5d band to 2π ∗ orbital. As CO moves to the hollow site at TS and FS, there are more antibonding states shifted above the Fermi level due to repulsive interactions with the Ir 5d electrons, see Figs. 10(b) and 10(c). The d- band center at TS and FS are −3.84 eV and −3.74 eV, respectively, which predicts weakening in the adsorbates bond according to the Hammer–Nørskov d-band model.40, 47
IV. CONCLUSIONS
The adsorption/absorption of carbon atoms on/in Ir(100) surface with different coverages as well as the diffusion of carbon atom in Ir(100) and the dissociation of CO molecule on Ir(100) surface are studied using periodic self consistent DFT-GGA calculations. The hollow site is found to be the most stable site for carbon atoms on Ir(100) surface at all coverages. We obtained a bridging metal structure for carbon atom on Ir(100) surface at 0.75 ML coverage due to the balance between Ir–C and Ir–Ir interactions. The octahedral site is found to be the most stable site for diffused carbon atom into the surface. The diffusion barrier of carbon atom is 2.70 eV, which is much larger than the segregation barrier (0.22 eV) from subsurface to the surface. Similarly, the energy barrier for CO dissociation on Ir(100) (3.01 eV) is larger than the association energy (1.61 eV). ACKNOWLEDGMENTS
This work was supported by the LinkSCEEM-2 project, funded by the European Commission under the 7th Framework Programme through Capacities Research Infrastructure, INFRA-2010-1.2.3 Virtual Research Communities, Combination of Collaborative Project and Coordination and Support Actions (CP-CSA) under Grant Agreement No. RI-261600. 1 Y.
Shamoto and M. Aoki, Mater. Trans. 47, 2674 (2006). E. Jiang and E. A. Carter, Phys. Rev. B 71, 045402 (2005). 3 A. T. N’Diaye, J. Coraux, T. N. Plasa, C. Busse, and T. Michely, New J. Phys. 10, 043033 (2008). 4 Q. Ge, D. A. King, N. Marzari, and M. C. Payne, Surf. Sci. 418, 529 (1998). 5 K. Heinz, G. Schmidt, L. Hammer, and K. Müller, Phys. Rev. B 32, 6214 (1985). 2 D.
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Baraldi, J. Cerdá, J. A. Martin-Gago, G. Comelli, S. Lizzit, G. Paolucci, and R. Rosei, Phys. Rev. Lett. 82, 4874 (1999). 7 P. Ghosh, S. Narasimhan, S. J. Jenkins, and D. King, J. Chem. Phys. 126, 244701 (2007). 8 D. Alfè, S. de Gironcoli, and S. Baroni, Surf. Sci. 437, 18 (1999). 9 K. Johnson, Q. Ge, B. Sauerhammer, S. Titmuss, and D. A. King, Surf. Sci. 478, 49 (2001). 10 A. de Koster and R. A. van Santen, Surf. Sci. 233, 366 (1990). 11 G. A. Somorjai, Introduction to Surface Chemistry and Catalysis (Wiley, New York, 1994). 12 W. M. H. Sachtler and M. Ichikawa, J. Phys. Chem. 90, 4752 (1986). 13 R. A. van Santen, I. M. Ciobîc˘ a, E. van Steen, and M. M. Ghouri, “Mechanistic, issues in Fischer–Tropsch catalysis,” Adv. Catal. 54, 127 (2011). 14 H. Pichler, Advances in Catalysis (Academic Press, New York, 1952), Vol. IV. 15 Z. Liu and, P. Hu, J. Chem. Phys. 114, 8244 (2001). 16 Z. Liu and P. Hu, J. Chem. Phys. 115, 4977 (2001). 17 Z. Liu and P. Hu, J. Am. Chem. Soc. 125, 1958 (2003). 18 I. A. Erikat, B. A. Hamad, and J. M. Khalifeh, Eur. Phys. J. B 67, 35 (2009). 19 S. Titmuss, K. Johnson, Q. Ge, and D. A. King, J. Chem. Phys. 116, 8097 (2002). 20 R. I. Masel, Principles of Adsorption and Reaction on Solid Surface (John Wiley, New York, 1996). 21 B. Hammer and J. K. Nørskov, Adv. Catal. 45, 71 (2000). 22 S. Dahl, A. Logadottir, R. C. Egeberg, J. H. Larsen, I. Chorkendorff, E. Törnqvist, and J. K. Nørskov, Phys. Rev. Lett. 83, 1814 (1999). 23 B. Hammer and J. K. Nørskov, Surf. Sci. 343, 211 (1995). 24 P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gertsmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, and R. M. Wentzcovitch, J. Phys.: Condens. Matter 21, 395502 (2009). 25 J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). 26 The US PP for iridium is taken from http://www.quantum-espresso.org/ pseudo.php.
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Kittel, Introduction to Solid State Physics, 6th ed. (Wiley, New York, 1986). 28 S. Riikonen, A. S. Foster, A. V. Krasheninnikov, and R. M. Nieminen, Phys. Rev. B 81, 125442 (2010). 29 D. S. Sholl and J. A. Steckel, Density Functional Theory: A Practical Introduction (John Wiley, New Jersey, 2009). 30 M. Methfessel and A. T. Paxton, Phys. Rev. B 40, 3616 (1989). 31 H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13, 5188 (1976). 32 I. A. Erikat, B. A. Hamad, and J. M. Khalifeh, Appl. Catal., A 449, 9 (2012). 33 H. W. Hugosson, O. Eriksson, U. Jansson, A. V. Ruban, P. Souvatzis, I. A. Abrikosov, Surf. Sci. 557, 243 (2004). 34 G. Henkelman, B. P. Uberuaga, and H. Jónsson, J. Chem. Phys. 113, 9901 (2000). 35 R. Terborg, J. T. Hoeft, M. Polcik, R. Lindsay, O. Schaff, A. M. Bradshaw, R. Toomes, N. A. Booth, D. P. Woodruff, E. Rotenburg, and J. Denlinger, Phys. Rev. B 60, 10715 (1999). 36 J. Ahn, H. Bu, C. Kim, V. Bykov, M. M. Sung, and J. W. Rabalais, J. Phys. Chem. 100, 9088 (1996). 37 S. Yamagishi, S. J. Jenkins, and D. A. King, J. Phys. Chem. C 112, 14417 (2008). 38 J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. A. Pederson, D. J. Singh, and C. Fiolhais, Phys. Rev. B 46, 6671 (1992). 39 P. Wu, W. Zhang, Z. Li, J. Yang, and J. G. Hou, J. Chem. Phys. 133, 071101 (2010) 40 B. Hammer and J. K. Nørskov, Phys. Rev. Lett. 79, 4441 (1997). 41 H. Chen, W. Zhu, and Z. Zhang, Phys. Rev. Lett. 104, 186101 (2010). 42 I. A. Erikat, B. A. Hamad, and J. M. Khalifeh, Phys. B 407, 468 (2012). 43 I. A. Erikat, B. A. Hamad, and J. M. Khalifeh, Phys. Status Solidi B 248, 1425 (2011). 44 I. A. Erikat, B. A. Hamad, and J. M. Khalifeh, Chem. Phys. 385, 35 (2011). 45 Y. R. Luo, Handbook of Bond Dissociation Energies in Organic Compounds (Science Press, Beijing, 2005), p. 209. 46 R. Zhang, L. Ling, Z. Li, and B. Wang, Appl. Catal., A 400, 142 (2011). 47 M. Mavrikakis, M. Bäumer, H.-J. Freund, and J. K. Nørskov, Catal. Lett. 81, 153 (2002).
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A first principle study for the adsorption and absorption of carbon atom and the CO dissociation on Ir(100) surface I. A. Erikat1,a) and B. A. Hamad2 1 2
Department of Physics, Jerash University, Jerash-26150, Jordan Department of Physics, The University of Jordan, Amman-11942, Jordan
(Received 9 July 2013; accepted 16 October 2013; published online 5 November 2013) We employ density functional theory to examine the adsorption and absorption of carbon atom as well as the dissociation of carbon monoxide on Ir(100) surface. We find that carbon atoms bind strongly with Ir(100) surface and prefer the high coordination hollow site for all coverages. In the case of 0.75 ML coverage of carbon, we obtain a bridging metal structure due to the balance between Ir–C and Ir–Ir interactions. In the subsurface region, the carbon atom prefers the octahedral site of Ir(100) surface. We find large diffusion barrier for carbon atom into Ir(100) surface (2.70 eV) due to the strong bonding between carbon atom and Ir(100) surface, whereas we find a very small segregation barrier (0.22 eV) from subsurface to the surface. The minimum energy path and energy barrier for the dissociation of CO on Ir(100) surface are obtained by using climbing image nudge elastic band. The energy barrier of CO dissociation on Ir(100) surface is found to be 3.01 eV, which is appreciably larger than the association energy (1.61 eV) of this molecule. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4827516] I. INTRODUCTION
The formation of graphene on transition metal surfaces is interesting to gain more information about the catalytic growth of single-wall carbon nanotubes (SW-CNT’s). Thus, the adsorption, diffusion, migration, and bond formation of carbon atoms on or in metallic catalyst should provide useful information to the manipulation and the design of carbonbased nanomaterials.1–3 It is well known in the literature that Ir(100) surface exhibits a quasi hexagonal reconstruction with (1 × 5) periodicity,4, 5 where the (1 × 1) unreconstructed overlayer is stable up to temperature >800 K.5 The adsorption of some small molecules, such as CO, NO, O2 , and H2 , can lift the reconstruction.6, 7 On the other hand, the adsorption of small adsorbates such as O, C, and N causes reconstruction.6, 8 Johnson et al.9 studied the structure of carbon adsorbed on Ir(100) surface using low energy electron diffraction and density functional theory. They found that carbon forms a c(2 × 2) overlayer and occupies the fourfold hollow site but does not cause the surface to clock reconstruct as found for Ni(100) by Alfe et al.,8 due to the iridium high cohesive energy. The dissociation of CO is very important in the production of hydrocarbons from carbon monoxide and hydrogen by Fischer-Tropsch (FT) synthesis and in the formation of higher oxygenates.10–12 Moreover, CO dissociation is the rate determining and the first step in many catalytic processes and is an essential step for getting rid of toxic substances that are produced during the burning of the fuel.13 It is wellknown that nickel, cobalt, iron, and ruthenium are very active as FT catalysts, while platinum, palladium, and iridium exhibit low activity.14 There are several studies for the case of a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +962 79 5238624.
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the active elements for CO dissociation, whereas, the studies for the less active catalysts are scarce. Liu and Hu15–17 performed DFT studies to calculate the dissociation of CO on series of flat, stepped, and kinked 4d as well as 5d closepacked metal surfaces, namely Ru(0001), Rh(111), Pd(111), Os(0001), Ir(111), and Pt(111) at 0.25 ML coverage. They found similar late transition states for all these surfaces. In addition, they found that the effect of decreasing the coverage to 1/9 ML is minor on the calculated energy barrier. For example, the energy barrier of CO dissociation on Rh(111) changed from 1.25 to 1.17 eV by decreasing the coverage from 1/4 ML to 1/9 ML.17 There are many experimental and theoretical studies for the adsorption of CO on Ir(100) surface, where CO is found to adsorb linearly (vertically) on Ir(100) surface with C atom pointing towards Ir surface. The top site is found to be the most stable site for coverages 0.25 ML and 0.50 ML.7, 18, 19 On the other hand, there are few studies in the literature about the dissociation of CO molecule on Ir surfaces.13, 15 It was found that the dissociation reactions are affected by two factors; the first one is the electronic factor, where the reactivity of transition metal dissociation reactions decreases from left to right in the Periodic Table.20, 21 The second factor is the geometrical one due to the relative change in the molecular-substrate interaction strength on low coordinated atoms.21–23 However, there are many details that remain unclear even after significant studies regarding the structure-sensitivity, the details of the reaction path, and the bonding coordination of reaction intermediates to the surface. The aim of the present study is to analyze the electronic factors that determine CO dissociation on Ir(100) surface. The motivation is twofold, one is environmental to reduce CO emission and the other is industrial to understand the effect of carburizing the Ir surface on its catalytic reactivity.
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II. CALCULATION METHOD
III. RESULTS AND DISCUSSION
We performed first-principles calculations based on DFT as implemented in Quantum espresso Package24 that solve Kohn-Sham equations using a plane-wave basis set. For the treatment of electron exchange and correlation, we use the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE).25 The electron-ion interactions are described with ultrasoft pseudopotentials.26 We use our previous calculated lattice constant, 3.899 Å,18 which is ∼1.7% higher than the experimental value (3.84 Å)27 to build our supercells. The surface is modeled using a 2 × 2 unit cell with a five-layer symmetric slab separated in the z-direction by a vacuum thickness of 15 Å. The adsorbates are placed on one side of the slab. The center of the slab consists of a mirror plane where the atoms are fixed at their bulk positions, whereas the layers above and below as well as the adsorbates are freely allowed to relax in all directions. The advantage of using this symmetric model is due to the negligible dipole correction28, 29 as we found a minor change of about 0.01 eV/C in the adsorption energy. A cut-off energy of 680.5 eV and Methfessel Paxton broadening with σ = 0.2 eV30 are used. The Brillouin zone is sampled with the Monkhorst-Pack k-mesh31 (6 × 6 × 1). The reference energy of the gas phase for C, O, at their triplet ground state, and CO are calculated by placing the atom or molecule in a cubic cell with a side length of 15 Å and carrying out a spin-polarized gamma-point calculations. The criteria for the converged calculations are 10−7 eV for the energy and less than 10−3 eV/Å for the maximum Hellmann-Feynman forces acting on each atom upon ionic relaxation. Carbon atoms are allowed to reach the surface at different surface sites (top, bridge, and hollow) and two different subsurface sites (tetrahedral and octahedral). The adsorption and absorption energies per carbon atom, Eads and Eabs , respectively, are both referenced to isolated carbon atoms in the gas phase and a pure iridium slab by the formula32
A. Carbon adsorption on Ir(100) surface
Eads/abs = −(Etot − EI r − mEC(g) )/m,
The adsorption of C atom on Ir(100) surface is investigated at the coverages 0.25 ML, 0.50 ML, 0.75 ML, and 1.00 ML. For all coverages the energetically most favorable adsorption site is the hollow site, which is consistent with previous studies of the adsorption of carbon atoms on (100) faces of other fcc transition metals such as Ni(100) and Rh(100).8, 35, 36 The adsorption energy of C on Ir(100) surface as a function of coverage is plotted in Fig. 1. The adsorption data indicates that C atoms bind strongly with Ir(100) surface preferring the high coordination hollow site. The adsorption energy decreases significantly for coverages greater than 0.25 ML, which indicates that a strong repulsive interaction builds up between adsorbates. The adsorption energy at 0.25 ML is 7.87 eV, which is more energetically stable than that for bridge and top sites by 1.14 eV and 2.31 eV, respectively. For this coverage at the hollow site, the Ir–C and Ir– Ir bond lengths are 2.06 Å and 2.78 Å, respectively, which means that the Ir–Ir distance beneath C atom increases by 0.02 Å from the original interatomic distance. In addition, Ir second layer atoms suffer from buckling of 0.06 Å. The outermost Ir atom at Ir(100)-C system with carbon atom at hollow site contracts by 0.59% relative to the unrelaxed clean surface interlayer spacing (d0 ). Moreover, the second layer contracts by 0.66%. However, by comparing this with the clean Ir(100)(2 × 2) surface,18 one can conclude that the adsorption of carbon atom on the hollow site at Ir(100)-C system decreases the contraction of the first layer by 5% and removes the expansion of the second layer. At 0.5 ML the hollow site is energetically the most stable site with adsorption energy of 7.73 eV, which is greater than those at the bridge and hollow sites by 1.11 and 2.16 eV, respectively. Our calculated adsorption energy at the hollow site is smaller by 0.46 than that obtained from LEED experiment (8.19 eV)9 and 0.33 eV from DFT calculations by Johnson et al. (8.04 eV)9 and Yamagishi et al. (8.06 eV).37 Table I represents a comparison between the energetic and
(2.1)
where Etot refers to the total electronic energy of the Ir(100) − mC system, EIr is the energy of clean Ir(100) surface, EC(g) refers to C electronic energy in the gaseous phase, and m is the number of C atoms on/in the surface. The generalized cohesive energy (Ecoh )33 is defined as Ecoh (AB) =
Etot (AB) − mA Eat (A) − mB Eat (B) , mA + mB
(2.2)
where Etot (AB) is the total energy of the solid and Eat (A, B) is the total energy of non-spin-polarized atoms. The effect of spin polarization is tested for all systems and found to be negligible. The climbing image-nudged elastic band (CI-NEB) method34 is used to locate the minimum energy paths (MEP) and the transition states.
FIG. 1. The adsorption/absorption energy per carbon atom as a function of C coverage (); top (t), bridge (b), hollow (h), and octahedral (oct).
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TABLE I. A comparison of the results in this work for the energetic and structural parameters of Ir(100)-(2 × 2)-2C system with LEED and previous DFT calculations.8, 39 Eads : the adsorption energy for carbon atom; Ir–C: iridium-carbon bond; dij : the relative interlayer distance between the ith and jth layers; z: second layer buckling; dIr–C : the vertical distance between the carbon atom and the first iridium layer.
This work DFTa LEEDa a
Eads (eV/C)
Ir–C (Å)
d12 (%) (%)
d23 (%)
z (Å)
dIr–C (Å)
7.73 8.04 8.19
2.06 2.04 2.05
3.53 4.40 2.70
−1.43 n/a −1.80
0.00 n/a 0.02
0.62 0.68 0.74
Reference 9.
structural properties in our work and previous experimental and DFT calculations of Ir(100)-(2 × 2)-2C system. The discrepancy between our calculations and the previous DFT calculations can be related to the difference between exchange correlation functionals used in both studies38 as well as the type of surfaces used. In our calculations we used the symmetric model for the surface, whereas those previous DFT calculations were performed using the asymmetric one, see Sec. II. Moreover, we did not include the reconstruction energy per (1 × 1) cell for the (1 × 5) →(1 × 1) phase transition of the clean surface as it is the case of Yamagishi et al.37 The Ir–C bond does not change by increasing the coverage and there is no buckling in the first and second layers. The outermost Ir layer expands by 3.5% relative to d0 , whereas the second layer contracts by 1.4% relative to d0 . At 0.75 ML coverage one of the three carbon atoms penetrates into the subsurface with two Ir–C bond lengths of 2.04 Å and 1.94 Å that creates bridge metal (BM) structure, see Fig. 2. This BM structure was also found to be spontaneously formed on Cu, Ag, and Ni surfaces.39 For this BM structure, we obtained an adsorption energy of 6.96 eV/C and a cohesive energy of 7.49 eV. Therefore, the relative strength (the ratio between the adsorption energy and the cohesive energy) is found to be 0.93, which is slightly smaller than that obtained by Wu et al. (1.00) for Ir(111) surface.39 However, we obtained a spontaneous BM structure at the Ir(100) surface unlike their findings for Ir(111). We can relate this to the fact that the Ir(100) surface is an open surface, whereas Ir(111) is the most compact surface for the fcc structure with smaller M-M bond
(a)
(b)
FIG. 2. (a) Top view and (b) side view of the BM structure at relaxed Ir(100)3C system.
length, see Fig. 2. The first layer is found to expand upwards by 7.3% relative to d0 with lateral motions of the atoms in the x and y directions away from each other by 0.04 Å. At the second layer, the iridium atoms below the carbon atoms move upward by 1.82% relative to d0 , whereas the fourth iridium atom is pulled upward by 9.6% relative to d0 . As the carbon coverage increases to 1.00 ML, the adsorption energy per carbon atom decreases to 5.99 eV and no carbon atoms penetrate into the surface unlike the case of the three monomer (0.75 ML). The existence of BM at 0.75 ML and not at 1.0 ML is related to the competition between Ir–C and Ir–Ir interactions. If the Ir–C interaction is stronger than that of Ir–Ir, the Ir atom below C will be up shifted, which leads to an increase of the coordination numbers of carbon atoms. This is not the case for the present system since Ir–Ir interaction is stronger than Ir–C. In order to confirm the validity of using the five-layer slab in the calculations, we used a larger slab of seven layers, where the three layers above and below the central layer are allowed to relax. We found that the adsorption energies for all coverages, with carbon atoms at the hollow site, have increased by 0.08 eV as compared to the values for the case of the five-layer slab. The percentage difference of the interlayer spacings with respect to the bulk values for the third layer is found to be negligible for all coverages. We also found that the effect of spin polarization is negligible for Ir(100)-C system at all coverages. Figure 3 represents the projected density of states (PDOS) for carbon atom adsorbed on the hollow site and the nearest neighbor Ir atom for the 0.25 ML, 0.50 ML, 0.75 ML, and 1.0 ML coverages. The adsorption of C on Ir(100) surface shifts the d-band center below the Fermi level relative to its location at clean surface by 0.40 eV, 0.85 eV, 1.24 eV, and 1.90 eV, for 0.25 ML, 0.50 ML, 0.75 ML, and 1.00 ML coverages, respectively. This means that the higher the d-band center relative to the Fermi level, the stronger the bonds between Ir and C atoms, which is consistent with the Hammer-Nørskov d-band model.40 The broadening of C 2p orbital increases with increasing the coverage due to the increase in the hybridization with Ir 5d orbital. There are overlaps between 2s + 2p for C atom and 5p + 5d for Ir atom from −14 to −11 eV. In the case of 0.25 ML coverage there is a strong hybridization between Ir 5d and C 2p orbitals near 2.20, 1.40, −7.60, −6.70, −5.30, −4.30, and −2.00 eV. As the coverage increases, the hybridization between d and p bands increases and the peaks in the antibonding region shift to the left. Moreover, for the 0.50 ML coverage, the peaks of Ir 5d orbital are well localized at −7.60, −7.00, −6.40, −5.30, −4.80, −4.3, −3.30, −2.67, 1.00, and 2.28 eV. This behavior indicates small repulsive interaction between the C atoms. In Fig. 3(d) (0.75 ML), one can notice the charge transfer from Ir 5d orbital to the 2p orbital of the carbon atom in BM structure, which decreases the density of state below the Fermi energy of the Ir 5d, while adding features to the energy of 2p level of the adsorbate (carbon atom). This leads to sharp peaks at −8.30, −7.63, −6.60, 1.47, 0.76, and 4.16 eV due to the strong hybridization between the 2p orbital of the carbon atom and Ir 5d. At the 1.00 ML coverage, the peaks become sharper at −12.10, −7.10, −6.60, −4.60, −3.8, −2.74, −1.70, 0.41, and 1.48 eV. From the above
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FIG. 3. (a) PDOS of the 5d orbital of Ir(100) clean surface. (b) PDOS of Ir (100)-C system with coverage 0.25 ML. (c) PDOS of Ir (100)-2C system with coverage 0.50 ML. (d) PDOS of Ir (100)-3C system with coverage 0.75 ML. (e) PDOS of Ir (100)-4C system with coverage 1.00 ML. Solid black line is Ir 5d, red dotted line is C 2p orbital for C atom at hollow site. The blue dashed line at (d) is C 2p for the BM carbon atom at Ir(100)-3C. The vertical dashed and solid lines denote the Fermi level and the center of the d band, respectively.
results we can argue that the adsorption of carbon atom on Ir(100) surface is coverage dependent.
B. Carbon absorption in Ir(100) surface
In this subsection we present the results of carbon absorption in Ir(100) surface. We find that carbon atom prefers the octahedral site, in the subsurface, see Fig. 4, with absorption energy of 5.39 eV, which is greater than that of the tetrahedral site by 0.53 eV. The absorption energy increases by 0.18, 0.42, and 0.49 eV when increasing the coverage to 0.50, 0.75, and 1.00 ML, respectively, see Fig. 1. From Fig. 1, one can see that at the 1.0 ML coverage, the difference between the adsorption energy at the hollow site is very close to the
FIG. 4. The most stable structure of C atom in Ir(100) subsurface. Ir atoms are in gray color and C atom is in yellow color.
absorption energy at the octahedral site. This means that the diffusion energy barrier at this coverage is very small. We also investigate the effect of increasing the carbon content of the (2 × 2) unit cell from one atom per unit cell up to four atoms and changing the relative amount of carbon on the surface and in the subsurface. In order to reduce the number of studied configurations, we consider only the configurations with highest adsorption/absorption energies (hollow site on the surface and octahedral in subsurface), see Fig. 5. This procedure can show the energetically preferred states and the energy barriers to move from one state to another. Figure 5 shows that the on surface position is favored for coverages up to 0.50 ML (2 carbon atoms per (2 × 2) cell). However, for 3 carbon atoms per (2 × 2) unit cell (0.75 ML coverage), the optimum structure is obtained when two of the three carbon atoms are located at the hollow sites and the third at the octahedral below the surface. The adsorption/absorption energy of this configuration is 7.08 eV/C, which is greater by 0.12 eV/C than the case with three carbon atoms adsorbed at the hollow sites. Moreover, in the case of four carbon atoms per (2 × 2) unit cell (1.00 ML coverage), the most stable structure is found when two carbon atoms are located at the hollow sites with an adsorption/absorption energy of 6.78 eV. The stability of this configuration is almost the same as the case when three carbon atoms are adsorbed at the hollow sites and the fourth in the octahedral site, with an energy difference less than 0.01 eV/C. The decrease of the stability at the higher coverages when all atoms are adsorbed on the surface can be explained by the increase of the repulsive interaction between carbon atoms on nearest neighbor sites.
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FIG. 5. The energy diagram of the adsorption/absorption energy per carbon atom on/in surface/subsurface of Ir(100)-(2 × 2). Cads represents adsorbed carbon atoms on the hollow site and Csub subsurface carbon on the octahedral site.
C. Carbon diffusion into Ir(100) surface
We used the CI-NEB method to obtain the minimum energy path of carbon diffusion into the Ir(100) surface. Figure 6 shows the minimum energy path of C diffusion into Ir(100) surface at 0.25 ML coverage. The energy change from the reactant to the product is 2.48 eV. The reaction is endothermic, which means that the carbon atom prefers to stay on the surface. We obtain a diffusion barrier of 2.70 eV going into the surface, while the reverse process has a barrier of 0.22 eV. Our finding for the diffusion barrier is much larger than the case of Ir(111) (0.75 eV).41 The reason of the high diffusion barrier obtained in our calculations as compared to the case of Ir(111) is referred to the higher difference between the adsorption and absorption energies of 2.48 eV/C versus
FIG. 6. The minimum energy path of C diffusion into Ir(100) and the structures for the initial, two intermediate, transition, and final states. Ir atoms are in gray and C in yellow.
1.78 eV/C for Ir(111).41 At the transition state, the carbon atom resides in an approximate tetrahedral site making two Ir–C bonds one with Ir surface atom and another with Ir in the second layer, see Fig. 6. Consequently, one Ir surface atom moves up towards the vacuum. According to the small energy difference of 0.53 eV/C between the subsurface at the tetrahedral site and the subsurface octahedral site, the diffusion of the carbon atom from the subsurface to the surface is rather easy.
D. The dissociation of CO molecule on Ir(100) surface
The adsorption of O and CO was studied extensively in our previous works.18, 42–44 At the 0.25 ML coverage, we found the bridge and top sites to be the most energetically stable sites with adsorption energies 5.20 eV and 2.37 eV, for O and CO, respectively. The dissociation energy for the free CO molecule is found to be 11.81 eV, which is in a fair agreement with the experimental value of 11.16 eV45 as well as previous GGA calculations of 11.31 eV.46 In this subsection, we investigate the dissociation of CO molecule on Ir(100) surface using the most strongly bound molecular state (top) as an initial state (IS) and the most stable final state (FS) for Ir(100)-(C + O) system, with C and O at the hollow and bridge sites, respectively. Seven intermediate images are suggested between the initial and final states to search for the reaction path using CI-NEB. Figure 7 shows the energy change along the minimum pathway and Fig. 8 shows snapshots of the initial, intermediate, and final states along the calculated minimum energy reaction path. The reaction is endothermic, with heat of reaction (H) of 1.40 eV between the IS and FS. The activation barrier (Ea ) of the dissociation reaction is 3.01 eV, which is large as compared to the barrier of the association reaction for C + O → CO (1.61 eV). The activation energies for CO dissociation depend strongly on the nature of the metal and
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FIG. 7. The minimum energy path for CO dissociation on Ir(100) surface as determined from CI-NEB calculations. Ea is the dissociation barrier and H is the heat of reaction.
is highly structure sensitive.13 Liu et al. found an activation energy of 1.47 eV for the CO reaction on Ir(111) surface15 by using constrained minimization method. In our previous study on CO oxidation on Ir(100) surface43 we found two different TSs and reaction paths by using constrained minimization and nudged elastic band methods. The energy barrier associated with TS, using the constrained minimization and NEB methods are 0.85 and 1.34 eV, respectively. The energy barrier obtained by constrained minimization method is smaller than that obtained by the NEB due to the different reaction paths of each method. van Santen et al. estimated the transition state energies for CO dissociation on several (111) metal surfaces
(a)
(f)
(b)
(j)
(c)
(h)
(d )
(e)
from the Brønsted–Evans–Polanyi formula and found an energy barrier of 3.48 eV (336 kJ/mol) at Ir(111) surface.13 The dissociation process starts when carbon monoxide at the top site inclines with a tilt angle of 2.34◦ . This inclination is expected because previous studies reported that perpendicularly adsorbed CO desorbs molecularly without dissociation at high temperatures,21 whereas the inclined CO is believed to dissociate due to strong interactions with the d-band of the substrate metals that causes the weakening of the C–O bond. At the second step carbon atom diffuses to the bridge site that has a smaller adsorption energy than the top site (2.20 eV) making two bonds with Ir atoms; 2.06 Å and 2.09 Å. Therefore, the C–O bond increases by 0.02 Å as compared to C–O bond at the IS (1.16 Å). At the next event, the carbon atom diffuses towards the hollow site and makes threefold bonding with the surface with Ir–C bond lengths about 2.17 Å. However, the fourth Ir atom has a larger Ir–C distance of 2.39 Å. At this metastable state, there is an increase in the C–O bond by 0.11 Å as compared to the same bond at IS, see Fig. 9(a). At this configuration, there is a buckling in the first layer as well as the second layer beneath CO molecule of
(i)
FIG. 8. Snapshots for the initial, seven intermediate, and final states along the dissociation energy path for CO on Ir(100) surface. (a) The initial state. (b), (c), (d), (e), (g), and (h) Intermediate images. (f) The transition state. (i) The final state. Dark gray balls represent Ir first layer atoms, light gray balls represent Ir second layer atoms, yellow balls represent C atoms, and red balls represent oxygen atoms.
(a)
(b)
FIG. 9. Side views CO dissociation reaction at (a) the intermediate state just before TS. (b) TS.
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FIG. 10. PDOS for three states at CO dissociation on Ir(100) surface: (a) IS, (b) TS, and (c) FS. The solid black line denotes Ir 5d band, the green dotted line denotes O 2p orbital, and red dashed line denotes C 2p orbital.
0.04 Å and 0.06 Å, respectively. At the transition state (TS), the carbon atom diffuses to the hollow site, which is the most stable site and oxygen atom moves to the nearest top site, see Fig. 9(b). At this state the three Ir–C bond lengths are 2.08 Å, 2.02 Å, and 2.02 Å, whereas, the Ir–O bond length is 1.88 Å. Finally, the carbon and oxygen atoms diffuse to the fourfold hollow and the bridge sites, respectively. The scenario of CO dissociation in our calculations has several features, first: CO diffuses from the most stable site (top) of adsorption energy of 2.37 eV to a less stable site (bridge) of adsorption energy of 2.20 eV and then to the most unstable site (hollow) of adsorption energy of 1.96 eV. As CO reaches the hollow site, the CO bond becomes weaker and the bond length increases by 0.11 Å. At TS, oxygen atom diffuses to the most unstable site, which is the top site. However, the carbon atom diffuses to the most stable site (hollow site). In addition, at TS, the distance between C and O is large, which is usually called ‘late TS.’’ This is consistent with the results found for other transition metal surfaces.15, 21, 22 The adsorption of CO molecule on Ir(100) surface lowers the d-band center from −2.7 eV to −3.52 eV. Figure 10 shows the distribution of the projected density of d-states of the Ir(100) surface as well as the p-states of C and O atoms. From Fig. 10(a), one can notice that there is a charge depletion from 5σ to Ir 5d band, also there is a back donation from Ir 5d band to 2π ∗ orbital. As CO moves to the hollow site at TS and FS, there are more antibonding states shifted above the Fermi level due to repulsive interactions with the Ir 5d electrons, see Figs. 10(b) and 10(c). The d- band center at TS and FS are −3.84 eV and −3.74 eV, respectively, which predicts weakening in the adsorbates bond according to the Hammer–Nørskov d-band model.40, 47
IV. CONCLUSIONS
The adsorption/absorption of carbon atoms on/in Ir(100) surface with different coverages as well as the diffusion of carbon atom in Ir(100) and the dissociation of CO molecule on Ir(100) surface are studied using periodic self consistent DFT-GGA calculations. The hollow site is found to be the most stable site for carbon atoms on Ir(100) surface at all coverages. We obtained a bridging metal structure for carbon atom on Ir(100) surface at 0.75 ML coverage due to the balance between Ir–C and Ir–Ir interactions. The octahedral site is found to be the most stable site for diffused carbon atom into the surface. The diffusion barrier of carbon atom is 2.70 eV, which is much larger than the segregation barrier (0.22 eV) from subsurface to the surface. Similarly, the energy barrier for CO dissociation on Ir(100) (3.01 eV) is larger than the association energy (1.61 eV). ACKNOWLEDGMENTS
This work was supported by the LinkSCEEM-2 project, funded by the European Commission under the 7th Framework Programme through Capacities Research Infrastructure, INFRA-2010-1.2.3 Virtual Research Communities, Combination of Collaborative Project and Coordination and Support Actions (CP-CSA) under Grant Agreement No. RI-261600. 1 Y.
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